Biologically active macromolecules such as antibodies specific for myosin, carcinoembryonic antigen (CEA), alpha feto protein or human chorionic gonadotropin labeled with radioactive iodine such as iodine-125 or iodine-131 have been used both experimentally and clinically to localize and visualize target organs. Radioactive iodine isotopes are not ideal isotopes for scintigraphy due to their half-lifes and their peak energy of emission. The peak energy of emission of iodine-125 at 35 kev is too low for scintigraphy and peak energy of emission of iodine-131 at 364 kev is too high. The presence of beta emission also make these two radioiodine isotopes undesirable for clinical use. An isotope that is better suited for scintigraphy is technetium-99m with peak energy emission at 140 kev, a half-life of 6 hours and no beta emission. Unfortunately, present techniques available for coupling technetium-99m to macromolecules are harsh on the macromolecules since the technetium is normally reduced and coupled in an acid environment which may result in degradation of the protein macromolecule. For example, technetium-pertechnetat is normally reduced in an acid pH to Tc.sup.+3 or Tc.sup.+4 with a stannous reducing agent. The protein macromolecule then is added to the reduced form of technetium-99m in order to radiolabel the protein molecule. At the acid conditions under which this radiolabeling is conducted, a significant amount of the protein becomes degraded thereby significantly reducing the amount of useful radiolabeled protein that can be obtained.
It would be highly desirable to provide a method for radiolabeling proteins or other biological molecules which minimized or prevented the degradation of the protein or biological molecule. Specifically, it would be desirable to provide a means for radiolabeling such molecules with technetium-99m, without accompanying denaturation of the biological activities.